1. Field of the Invention
This invention relates to a method of refining metal, for example, iron or its alloys to a high degree of purity in an ultra-high vacuum and a vacuum induction melting apparatus for putting the method into practice in order to prevent oxidation due to oxygen and water vapor which causes problems when metal of high purity is melted.
2. Background
Studies on metal of ultra-high purity have steadily progressed in recent years. In order to better understand the properties of these metals, there is a need for forming these metal materials of ultra-high purity into usable shapes and establishing the technology of refining metal to a ultra-high degree of purity. The term metal in this description means metal or metal alloy.
For example, a vacuum induction melting method, a floating zone melting method, etc. have heretofore been used for refining metal. These methods employ an electron beam, high-frequency induction energy and the like as a heat source. In particular, attention has been directed to the latter method as being advantageous in that there is no possibility of allowing impurities to penetrate through a refractory when the metal is melted.
Recently, there has been proposed an electron-beam, floating zone melting method combining both the aforementioned methods, and the processing is normally performed in a vacuum of about 10.sup.-6 Torr therein. Iron of the highest purity obtained according to the conventional methods usually has a resistance ratio of about 3,000.
Furthermore, studies on the technology of refining metal to a high degree of purity using a vacuum induction melting apparatus have steadily progressed in recent years. Such vacuum induction melting apparatus generally contain a high-frequency induction melting type crucible, that is, a high-frequency induction heating type crucible.
Each of those crucibles includes a crucible or crucible body made of refractory material such as alumina, magnesia and calcia, and an induction heating coil wound around the crucible at predetermined intervals. The crucible is made of refractory material. Metal in the crucible is melted by applying a high-frequency current to the coil around the crucible so as to utilize an induction eddy current flowing through the metal therein and the resistance of the metal.
There has been proposed a method of refining metal to a high degree of purity by a vacuum induction melting apparatus for heating metal by high-frequency induction heating in a crucible made of refractory material and placed in a high-vacuum vessel.
FIG. 1 shows a high-frequency vacuum induction melting apparatus which is used when metal is refined to a high degree of purity.
In this apparatus, a crucible 102 made of refractory material is placed in a vacuum vessel 101, and a high-frequency induction heating coil 103 used for induction-heating the metal to be melted in the crucible is fixedly mounted around the crucible 102.
In the vacuum induction melting apparatus, the crucible 102 placed in the vacuum vessel 101 is generally made by sinter-forming a metal oxide (Al.sub.2 O.sub.3, MgO, CaO, etc.) and the induction heating coil 103 is wound around the crucible. Further, a high-frequency current is supplied to the induction heating coil 103 so as to melt a material (metal) in the crucible 102 by induction heating.
The induction heating coil 103 is formed such that the exterior of a water-cooled copper pipe is insulated by an asbestos or glass tape and the high-frequency current can be supplied from an external high-frequency power supply 105 to the induction heating coil 103 through the water-cooled cable 104.
A portion of the vacuum vessel 101 through which the water-cooled cable 104 passes has an insulation-vacuum seal structure, and a flange portion including a door. The vacuum seal portion thereof is vacuum sealed with an O-ring (e.g., of fluoro rubber).
In this apparatus, vacuum is achieved by a vacuum pump 106 combining an oil diffusion pump, an oil rotary pump and a mechanical booster pump together, and the attainable degree of vacuum therein is generally about 10.sup.-5 Torr, although this varies with the influence of the gas discharged from the crucible 102 made of refractory material, the vacuum vessel 101, the water-cooled cable 104 and so on.
In the use of the aforementioned conventional apparatus, the crucible made of refractory material causes inorganic compounds such as magnesia and alumina to be extracted from the surface of the crucible into the liquid metal. This increases the concentration of impurities all the more. This phenomenon unavoidably occurs when metal is vacuum-refined to a high degree of purity and makes it difficult to maintain the purity of the original material.
More specifically, in the use of the aforementioned conventional art, there has been the following problems, particularly in the case of refining metal based on the high-vacuum induction heating technology:
(1) The use of a crucible made of refractory material causes inorganic compounds to be extracted out of the crucible into mother liquid metal, and the mixture of O.sub.2 as well as the desorption of adsorption gas (H.sub.2 O, N.sub.2, O.sub.2) hinders achieving a ultra-high degree of purity;
(2) Organic substance (hydrocarbon) and moisture are produced from the water-cooled cable and heating-coil insulating material, which results in an atmosphere rich in H.sub.2 O;
(3) The penetration of O.sub.2 and CO.sub.2 through the O-ring seal portion causes oxidation of the liquid metal.
Consequently, a rise in the operating pressure (degree of vacuum) ensues and particularly melting occurs in an atmosphere where the partial pressure of each of O.sub.2, H.sub.2 O and CO.sub.2 is high, thus causing metal contamination. In the case of Fe, for example, the partial pressure of O.sub.2 in the atmosphere needs to be 1.times.10.sup.-10 Torr or lower in order to make the concentration of O.sub.2 in liquid iron 10 ppm or lower; however, it has not been possible to achieve that degree of vacuum in the aforementioned conventional art.
Moreover, the introduction of impurities from the refractory material of the crucible make it difficult to maintain the purity of the original material by the use of the conventional method of refining metal to a high degree of purity in the conventional high-frequency vacuum induction melting apparatus.
As set forth above, the conventional technology of refining metal to a high degree of purity is problematic in that it is incapable of achieving a high degree of purity due to the limitation of the refining method and the unsatisfactory apparatus but also apt to increase the concentration of impurities in the metal.